We do not know all of the causes of ASD. However, we have learned that there are likely many causes for multiple types of ASD. There may be many different factors that make a child more likely to have an ASD, including environmental, biologic and genetic factors.
Most scientists agree that genes are one of the risk factors that can make a person more likely to develop ASD.4 In many families, there appears to be a pattern of autism or related disabilities, further supporting the theory that the disorder has a genetic basis. While no one gene has been identified as causing autism, researchers are searching for irregular segments of genetic code that children with autism may have inherited. It also appears that some children are born with a susceptibility to autism, but researchers have not yet identified a single “trigger” that causes autism to develop. Other researchers are investigating the possibility that under certain conditions, a cluster of unstable genes may interfere with brain development, resulting in autism. Still other researchers are investigating problems during pregnancy or delivery as well as environmental factors such as viral infections, metabolic imbalances and exposure to chemicals.
Children who have a sibling with ASD are at a higher risk of also having ASD.
Autism tends to occur more frequently than expected among individuals who have certain medical conditions, including fragile X syndrome, tuberous sclerosis, congenital rubella syndrome and untreated phenylketonuria (PKU). Some harmful substances ingested during pregnancy also have been associated with an increased risk of autism.
Research indicates other factors besides the genetic component are contributing to the rise in increasing occurrence of autism – for example, environmental toxins (e.g., heavy metals such as mercury), which are more prevalent than in the past. Those with autism (or those at risk) may be especially vulnerable to such toxins, as their ability to metabolize and detoxify these exposures might be compromised.
When taken during pregnancy, the prescription drugs valproic acid and thalidomide have been linked with a higher risk of ASD.
There is some evidence that the critical period for developing ASD occurs before, during, and immediately after birth.
Children born to older parents are at greater risk for having ASD. ASD continues to be an important public health concern. Like the many families living with ASD, CDC wants to find out what causes the disorder.
Understanding the factors that make a person more likely to develop ASD will help us learn more about the causes. We are currently working on one of the largest U.S. studies to date, called Study to Explore Early Development (SEED). SEED is looking at many possible risk factors for ASD, including genetic, environmental, pregnancy, and behavioral factors.